Re: [webcomponents] defined promise (#427)

I'd prefer having `customElements` attribute on `document` or `window` instead of adding a bunch of methods on `document` as in:

```js
document.customElements.define('my-element', MyElement);
document.customElements.check('my-element').then(~);
```


---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/427#issuecomment-194658888